In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J.Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N.ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading, resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to utilize an applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, which adaptively weights and combines the received signals fed to the base station, optionally utilizing additional antennas. FIG. 1 shows a base station utilizing an applique. A key to the viability of utilizing the applique approach is that it should require little, if any, modification of the base station equipment. This constraint implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
A difficulty in obtaining transparency to the existing equipment is the delay introduced by the signal processing performed in the applique: data acquisition, weight calculation, and received signal combining all introduce significant delay. Although typical cellular base station receivers are capable of accommodating some delay due to signal propagation, such delays are typically limited to the order of tens of microseconds, whereas the delay typically required to determine the weights for optimum array combining is many times that. Therefore, because of the applique delay, the signal seen by the existing base station receiver would appear to have had a propagation delay far in excess of the base station's ability to compensate, and the system would not operate.
A previously proposed solution is to use RF analog weighting and combining of the received signals, rather than digital signal processing of the received signals. However, although RF analog weighting introduces negligible processing delay into the signal path through the applique, the processing time required to calculate the weights is not negligible. The delay to calculate the weights is undesirable because a degraded signal will be output during the time prior to computation of the weights and the array performance will be poor when the fading rate is faster than a few Hz, which is the case at typical vehicle speeds in mobile communications.